The semiconductor industry uses masks for photolithography techniques to form microscopic or sub-microscopic circuit elements such as integrated circuits. In photolithography a substrate, e.g., a semiconductor wafer is covered with a photoresist that reacts to exposure to radiation. Radiation from a source is focused onto the photoresist through a patterned mask, known as a reticle. In semiconductor pattern generation, the reticle is typically a glass or quartz substrate bearing the image of an integrated circuit. A reticle typically has a mask substrate, a patterned layer and a protective covering layer known as a pellicle. The pattern on the reticle corresponds to a portion or layer of the desired integrated circuit. Portions of the photoresist that are exposed to the radiation react with light such that they are either easily removed (for a positive resist) or resistant to removal (for a negative resist), e.g., by a solvent. After removal of portions of the resist, a reduced image of the pattern is transferred to the photoresist. Portions of the wafer may then be etched through openings in the pattern on the photoresist. Alternatively, material may be deposited on the wafer through the openings in the photoresist. The size of the features on the photoresist pattern is limited by diffraction. As successive generations of integrated circuits require smaller and smaller circuit features, shorter wavelengths of radiation must be used. The use of shorter wavelengths can have an undesirable impact on the material used as the mask substrate.
Amorphous fused silica (silicon dioxide) is widely used for transparent mask substrates for photolithography. Fused silica is also likely to be used for reflective mask substrates, particularly for extreme ultraviolet (EUV) lithography. Unfortunately, silicon dioxide surfaces can be highly reactive to 248-nm, 193-nm and 157-nm radiation in the presence of water. Reactions involving the silicon dioxide, water vapor and radiation can produce crystal growth on the surfaces of fused silica substrates. This crystal growth can cause patterning errors on semiconductor wafers. In extreme cases crystal growth can be so dense on the backside of the reticle that it can cause a global transmission loss through the reticle which can result in a global change of CD (critical dimensions) of lines on the wafer.
Calcium Fluoride (CaF2) is a crystalline material that has been proposed for future 157-nm photolithography applications due to its preferable transmission characteristics at that wavelength. CaF2 coatings and magnesium fluoride (MgF2) coatings have been used on lenses in scanner optics as anti-reflective coatings, but not on photomasks. The thickness of these anti-reflective coatings is ¼ wavelength (e.g. 50 nm to 80 nm depending on DUV or i-line optics). CaF2 masks were proposed since they provide a substantially water-free environment and are less susceptible to undesired crystal growth than silica-based substrates. Unfortunately, photo masks for 157-nm photolithography require a quartz pellicle since the 157-nm radiation tends to destroy the polymers commonly used as a pellicle material. Thus CaF2 masks for 157-nm photolithography are relatively expensive compared to conventional photo masks. Development programs for 157-nm lithography have been stopped and engineering development resources focused on 193-nm immersion photolithography using fused silica mask substrates. Examples of masks for photolithography made using CaF2 as a mask substrate are described in commonly-assigned U.S. patent application Ser. No. 11/075,993 to William Volk et al. and entitled “USE OF CALCIUM FLUORIDE FOR LITHOGRAPHY MASKS,” which is incorporated herein by reference in its entirety. Although CaF2-based photo-masks are advantageous in terms of susceptibility to crystal growth, fused silica is more advantageous in terms of cost and transmission properties for photolithography at 248-nm, 193-nm and 157-nm wavelengths.
Thus, there is a need in the art, for lithography masks characterized by relatively low cost, desirable optical transmission and resistance to undesired crystal growth.